Accounting For Corrosion Of Hlw Glasses By Humid Air In TSPA
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ACCOUNTING FOR CORROSION OF HLW GLASSES BY HUMID AIR IN TSPA W. L. Ebert, J. C. Cunnane, and N. L. Dietz Argonne National Laboratory, Argonne, IL 60439 ABSTRACT This paper describes how the results of vapor hydration tests (VHTs) are used to model the corrosion of waste glasses exposed to humid air in the glass degradation model for total system performance assessment (TSPA) calculations for the proposed Yucca Mountain disposal system. Corrosion rates measured in VHTs conducted at 125, 150, 175, and 200°C are compared with the rate equation for aqueous dissolution to determine parameter values that are applicable to glass degradation in humid air. These will be used to determine the minimum for the range and distribution of parameter values in calculations for the Yucca Mountain disposal system license application (TSPA-LA). The rate equation η• for glass dissolution is rate = kE • 10 pH • exp(–Ea/RT). Uncertainties in the calculated rate due to the range of waste glass compositions and water exposure conditions are taken into account by using a range of values for the rate coefficient kE. The parameter values for the pH dependence (η) and temperature dependence (Ea) and the upper limit for kE are being determined with other tests. Using the values of η and Ea from the site recommendation model, the VHT results described in this paper provide a value of log kE = 5.1 as the minimum value for the rate expression. This value will change slightly if different pHand temperature-dependencies are used for the TSPA-LA model. INTRODUCTION Total System Performance Assessment (TSPA) calculations conducted as a part of license application for the proposed high-level radioactive waste disposal system at Yucca Mountain will be used to evaluate the impact of breached waste packages on the system performance during the 10,000-year regulated service life of the disposal system. The submodel for waste glass degradation will provide radionuclide release rates based on the glass dissolution rate and the radionuclide inventory as a source term for other models that simulate radionuclide transport and release from the disposal system. The glass dissolution rate will be called for during model runs at specific temperature and pH values. It will be calculated by the glass degradation submodel as the product of temperature and pH terms and a rate coefficient, which accounts for the effects of glass composition and the composition of the solution contacting the glass. The rate coefficient will be selected stochastically from a range and distribution of values that account for the range of waste glass compositions and the uncertainties in both the form of the dissolution rate equation and the parameter values. The range of the rate coefficient values will also be used to account for uncertainty in whether glass is contacted by humid air, dripping water, or is immersed in water, and the distribution will account for the likelihood that a particular condition will occur. Condensation of humid air is the most plausible pathway by which
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